This is a real tricky one, both B & C are very close.
Yet, I would go for option C.
Option B - The passage never states that measurement of acidity is the only factor of determining quality of unrefined olive oil. Hence B does not stand fully true.
Option C - The passage however mentions that the amount of free oleic acid an unrefined olive oil contains is directly proportional to its quality. It also mentions that amount of free oleic acid can determine olive oils acidity. By linking the two statements C can be concluded:
"If an unrefined olive oil is intermediate in acidity between two other unrefined olive oils, it will also be intermediate between them in quality. "